March 14 - Random sampling error o Sampling error or bias...

March 14, 2008Random sampling erroroSampling error or bias: the difference between the sample value and the true population value:Random: due solely to chanceSystematicsoStatisticians estimate the probability that a sampling result is representative of the population using sampling distributionSystematic sampling erroroRefers to differences between the sample and population due to some problem with the sampling methodoMajor sources of systematic error includeBad sampling frames Non responseBad sampling framesoGaps in a sampling frame will generate a sample which is not representative of the populationNon responseoRefers to elements selected from the sample from which no data are collectedE.g. people who hung up in telephone surveyoNon response can bias the sample if non responders are systematically different from those who do respondE.g. regular drug users may not respond to survey questions on drug use, yielding a sample of people who don’t use drugs as often as in the overall populationResult would be an underestimate of useoNon response rates > 30% indicates serious trouble, ability to generalize to population is seriously compromised.Combat systematic error by…oCareful evaluation and correction of sampling framesoUse of cluster sampling or other “frameless” methodsoMinimizing non response through repeated efforts to get data from non

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